Many industrial processes require energy for one or more process steps. A typical energy demand is for material drying or water evaporation. Most industrial processes use natural gas or fuel oil for process heat or steam production. Natural gas and fuel oil are premium fuels whose prices fluctuate significantly and are also high-priced. Solid fuels such as coal are a low-cost alternate that can be used is several cases for the same purpose. However, there are operational challenges with using coal, either via combustion or gasification, because of the increased carbon dioxide emissions relative to natural gas and also because of the ash in the coal that causes operational problems such as deposition and fouling of heat transfer surfaces. A new way of using coal for process heating applications or steam generation is required, which will minimize operational issues while at the same time minimize the increase of the carbon footprint (carbon dioxide emissions).
Additionally, the use of solid fuel for energy generation, while resulting in lower operating costs, requires equipment that is more expensive than when using clean fuels such as natural gas. When using a solid fuel, a higher return on capital investment is required via the simultaneous generation of alternate products that carry a higher economic value than just energy supply.
Several carbon-rich products have economic value and end-use applications. These include but are not limited to the following: activated carbon and activated charcoal for various gas cleaning and liquid processing applications; carbon-rich solids that can be used for soil amendment or as carriers of fertilizing compounds for slow-release into the environment; ultra-high surface area carbons for ultra-capacitors; and porous carbons for gas storage or gas separation. The activated carbon used for such applications is manufactured from hydrocarbon materials like coal or coconut shells.
Manufacturing of these carbon-rich products is typically from a hydrocarbon source such as coal or biomass using the steps of pyrolysis (heating in a non-oxidizing environment) and/or further activation (such as reaction with steam at high temperature to increase porosity or surface area). For example, activated carbon with a surface area of 400 m2/g or greater can be produced from lignite coal via the steps of pyrolysis at 450 to 650° C. and reacting with steam at temperatures between 750 and 1000° C. The production of such carbon-rich solids from parent hydrocarbons results in a hydrogen-rich product gas (a mixture of various compounds, in sum, having an elemental H/C ratio more than that of the parent hydrocarbon) that is typically not fully utilized in the manufacturing of the carbon-rich solid. For example a very large fraction of the hydrogen-rich product gas in activated carbon production is burnt and then quenched to reduce the flue gas to an adequate temperature for the gas cleaning apparatus, cleaned, and then exhausted into the environment. Such operation is not efficient and results in emission of pollutants including carbon dioxide that are excessive.
In existing activated carbon production plants, a hydrocarbon material like coal or biomass is typically processed through the steps of (i) drying, (ii) carbonization, and (iii) steam activation (contacting with steam at temperatures greater than about 800° C. to partially gasify the carbonized material and increase its surface activity). These steps can be performed separately, for example, in separate rotary kilns. They can also be performed in one reactor such as a multiple hearth furnace. Instead of steam, carbon dioxide can also be used in the activation step. Both the carbonization and activation steps generate combustible gases (hydrogen-rich product gas). These gases are exhausted from the activation carbon production furnace into a separate combustion chamber where they are oxidized with air to mainly carbon dioxide and water vapor before being sent to an air pollution control system to remove pollutants such as sulfur dioxide and particulate. Steam for the activation step is typically generated in the combustion chamber with a heat exchanger.
The combustible gases from the carbonization and activation steps typically contain condensable tars, which are the product of coal devolatilization. If there are locations in the process equipment that are colder than the condensation temperature, the condensed tars can cause deposits and fouling of the process equipment. The tars will remain in the gaseous phase if the temperature is maintained above their condensation point, which typically ranges between 300-650° F. For the above reason, the combustible gases from the carbonization and activation steps have to be maintained at relatively high temperatures, pressurized and transported using blowers which consume substantial energy, and combusted in a burner/combustion chamber that needs to be physically closely coupled to the carbonization and activation reactors.
Up to 1 pound of steam per pound of feed coal may be required for the activation furnace or about 1000-1200 Btu per pound of feed. This only represents about one-fifth the energy in the combustible gases. In current generation plants, the remainder of the energy is wasted, resulting in a combustible gas energy utilization of only about 20%. For example, the gases are cooled with a water quench before being directed to an air pollution control system. Greater than about 60 percent of the heat in the original starting material for the production of activated carbon is exhausted into the environment without beneficial use.
Alternatively, in US Patent Application Publication No. 20070254807, an elaborate and expensive steam-to-electricity system is added on to extract some of the energy from the combustible gas into a useful product. The efficiency of conversion to electricity in such plants is only about 25 percent of the energy in the combustible hot gases leaving the activated carbon production process. Also a significant amount of equipment and expense is required to set up the power plant, including steam production heat exchangers (boiler), steam turbines and condensers. A major portion of the heat is exhausted to the environment in the condenser section, where the low pressure steam is contacted with cooling water to condense it before its return to the boiler. The cooling water is then cooled in a cooling tower and heat rejected to the environment before being returned to the condenser. The low energy utilization occurs because only the expansion energy associated with the high temperature, high pressure steam is used in a steam turbine and the latent heat of evaporation associated with the water is rejected to the environment.
Yet another potential use for the combustible gases from the production of activated carbon from coal or biomass is its conversion to liquid fuels or chemicals. However, the composition and purity (cleanliness) requirements for the conversion of the combustible gases from the activated carbon production furnace to fuels and chemicals are very demanding. A narrow range of molecular hydrogen (H2) and carbon monoxide (CO) ratios are required. No diluents, such as nitrogen, can be used, requiring the use of pure oxygen in the generation of the combustible gases. The gases also need to be treated and cleaned to remove many contaminants such as sulfur compounds and ash to prevent poisoning of catalysts in downstream processing equipment before their conversion to liquid fuels or chemicals. The application to convert to liquid fuels or chemicals is therefore very expensive and requires extensive equipment and many processing steps.